Sh4dowI haven't skied in Washington but I have done a lot of research on Washington ski areas because I am considering living there at some point. If you are looking for big mountain gnarly terrain, it pretty much comes down to Baker, Stevens, Alpental, and Crystal. Baker is generally considered the best, but a lot of their good terrain is lift-accessed (sometimes wtih a short hike) but out of bounds, meaning you need avy training/gear/partner. It is my understanding that Baker is a great (though relatively small) in bounds ski area as well, but while it is generally considered the best in Washington for gnarly big mountain terrain, I don't know if that still holds true when the out-of-bounds is not factored in. It's a few hours from Seattle, a bit farther from the other three good places.
As for Stevens Pass, Alpental, and Crystal Mountain, I would assume that they're all pretty equal, there isn't really a conensus on which one is best.
Alpental and Crystal have lift serviced backcountry, but Baker is the only place I have ever had patrol stop me and be like, if you don't have a shovel/probe/beacon etc you cannot ski here. Alpental is pretty much a shitshow when it comes to backcountry access. You have snowboarders postholing up the traverse, no one has any b/c gear or knowledge, there's 30 people packed together on a traverse under a cornice that they don't even realize exists. Its where n00bs go for BC access.